This invention relates generally to steam turbine buckets (or blades) and, more particularly, to composite blades designed to provide different predetermined temperature capabilities in different selected areas of the airfoil portions of the blades.
For turbine buckets or blades, centrifugal loads are a function of the operating speed, the mass of the blade, and the radius from engine centerline where that mass is located. As the mass of the blade increases, the physical area or cross-sectional area must increase at lower radial heights to be able to carry the mass above it without exceeding the allowable stresses for the given material. This increasing section area of the blade at lower spans contributes to excessive flow blockage at the root and thus lower performance. The weight of the blade contributes to higher disk stresses and thus to potentially reduced reliability.
Several prior U.S. patents relate to so-called “hybrid” blade designs where the weight of the airfoil is reduced by composing the airfoil as a combination of a metal and polymer filler material. Specifically, one or more pockets are formed in the airfoil portion and filled with the polymer filler material in such a way that the airfoil profile is not altered. These prior patents include U.S. Pat. Nos. 6,139,278; 6,042,338; 5,931,641 and 5,720,597. See also co-pending and commonly owned application Ser. No. 10/249,518 filed Apr. 16, 2003. The '518 application discloses hybrid blades where pocket configurations are altered to vary the damping characteristics of respective groups of blades.
Another issue relating to the use of hybrid steam turbine blades, however, relates to cost as a function of temperatures experienced by such blades during use. In a double flow steam turbine, for example, there is significant windage heating of the last stage blade tip area during partial load and full speed conditions. The hood area behind the blades has a water spray system to cool the exhaust flow to the condenser. Even during the operation of the water sprays, however, the cooling flow does not migrate to the heated area near the blade tips, and thus, cooling of the blade tips is minimal. The blade tips during this condition can reach in excess of 400° F. wherein, during normal operation, the blade temperatures reach only about 150° F. Accordingly, most of the current polymers (urethanes and/or rubbers) considered for hybrid bucket applications are restricted for use at less than 300° F. While there are a few high temperature polymers available, their cost may be as much as 5× the cost of the lower temperature polymers.